US20240417330A1 - Boehmite structure and method for producing same - Google Patents

Boehmite structure and method for producing same Download PDF

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US20240417330A1
US20240417330A1 US18/700,746 US202218700746A US2024417330A1 US 20240417330 A1 US20240417330 A1 US 20240417330A1 US 202218700746 A US202218700746 A US 202218700746A US 2024417330 A1 US2024417330 A1 US 2024417330A1
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boehmite
particles
hydraulic alumina
boehmite structure
inorganic oxide
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Naoki Kurizoe
Ryosuke Sawa
Natsuki Sato
Tatsuro YOSHIOKA
Tohru Sekino
Tomoyo Goto
Sunghun CHO
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Panasonic Intellectual Property Management Co Ltd
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Panasonic Intellectual Property Management Co Ltd
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Assigned to PANASONIC INTELLECTUAL PROPERTY MANAGEMENT CO., LTD. reassignment PANASONIC INTELLECTUAL PROPERTY MANAGEMENT CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: CHO, SUNGHUN, GOTO, TOMOYO, SEKINO, TOHRU, KURIZOE, NAOKI, SATO, NATSUKI, SAWA, RYOSUKE, YOSHIOKA, Tatsuro
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Definitions

  • the present invention relates to a boehmite structure and a method for producing the boehmite structure.
  • Boehmite is an aluminum oxide hydroxide represented by a formula of AlOOH. Boehmite is insoluble in water and hardly reacts with acids and alkalis at normal temperature, and thus has high chemical stability and also excellent heat resistance due to its high dehydration temperature of around 500° C.
  • the powder of boehmite having such properties is used as a resin additive, a catalyst raw material, and an abrasive.
  • Boehmite has a specific gravity of about 3.07. It is thus awaited to develop a structure that is lightweight and excellent in chemical stability and heat resistance by using boehmite.
  • Patent Literature 1 discloses that a porous boehmite molded body is obtained through hydrothermal treatment of a mixture made from an aluminum hydroxide, a reaction promoter, and water at a temperature of 140° C. to less than 350° C.
  • porous boehmite molded body plate-like or needle-like boehmite crystals have a continuous crystalline structure and are connected to each other to form continuous pores, and the porous boehmite molded body has a porosity of 65% or more and has a flexural strength (JIS R1601) of 400 N/cm 2 or more.
  • Patent Literature 1 Japanese Unexamined Patent Application Publication No. 2003-238150
  • Patent Literature 1 describes that the flexural strength of the molded body obtained is about 1,600 N/cm 2 (16 MPa) at the maximum. However, when the molded body is used as a plate member, for example, a higher strength is required.
  • An object of the present invention is to provide a boehmite structure having high strength, and to provide a method for producing the boehmite structure.
  • a boehmite structure includes a plurality of boehmite particles where adjacent boehmite particles are bonded to each other.
  • a boehmite crystallite size is 10 nm or less, and a porosity is 15% or less.
  • a method for producing a boehmite structure according to a second aspect of the present invention includes a mixing step of obtaining a mixture by mixing mechanochemically treated hydraulic alumina with a solvent including water, and a pressure heating step of pressurizing and heating the mixture under a condition of a pressure of 10 to 600 MPa and a temperature of 50 to 300° C.
  • FIG. 1 is a schematic cross-sectional view of an example of a boehmite structure according to a present embodiment.
  • FIG. 2 is a schematic cross-sectional view of another example of the boehmite structure according to the present embodiment.
  • FIG. 3 is a diagram illustrating the flexural strength and Vickers hardness of test samples of example 1-1 and comparative example 1-1, and the results of observing the surfaces of the test samples using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • FIG. 4 is a graph illustrating X-ray diffraction patterns of test samples of examples 2-1 and 2-2, and comparative example 2-1, and X-ray diffraction patterns of boehmite (AlOOH) and nordstrand (Al(OH) 3 ) registered in ICSD.
  • FIG. 5 ( a ) is a diagram illustrating a reflected electron image at position 1 in test sample 1 of example 3-1.
  • FIG. 5 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 1 in test sample 1 of example 3-1.
  • FIG. 6 ( a ) is a diagram illustrating a reflected electron image at position 2 in test sample 1 of example 3-1.
  • FIG. 6 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 2 in test sample 1 of example 3-1.
  • FIG. 7 ( a ) is a diagram illustrating a reflected electron image at position 3 in test sample 1 of example 3-1.
  • FIG. 7 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 3 in test sample 1 of example 3-1.
  • FIG. 8 ( a ) is a diagram illustrating a reflected electron image at position 1 in test sample 1 of example 3-2.
  • FIG. 8 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 1 in test sample 1 of example 3-2.
  • FIG. 9 ( a ) is a diagram illustrating a reflected electron image at position 2 in test sample 1 of example 3-2.
  • FIG. 9 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 2 in test sample 1 of example 3-2.
  • FIG. 10 ( a ) is a diagram illustrating a reflected electron image at position 3 in test sample 1 of example 3-2.
  • FIG. 10 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 3 in test sample 1 of example 3-2.
  • FIG. 11 ( a ) is a diagram illustrating a reflected electron image at position 1 in test sample 1 of example 3-3.
  • FIG. 11 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 1 in test sample 1 of example 3-3.
  • FIG. 12 ( a ) is a diagram illustrating a reflected electron image at position 2 in test sample 1 of example 3-3.
  • FIG. 12 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 2 in test sample 1 of example 3-3.
  • FIG. 13 ( a ) a diagram illustrating a reflected electron image at position 3 in test sample 1 of example 3-3.
  • FIG. 13 ( b ) is a diagram illustrating binarized data of the reflected electron image at position 3 in test sample 1 of example 3-3.
  • FIG. 14 illustrates photographs of the results of observing particles of hydraulic alumina of example 4-1 and comparative example 4-1 at magnifications of 500 times and 3,000 times.
  • FIG. 15 is a graph illustrating the results of infrared spectroscopic analysis for the hydraulic alumina of example 4-1 and comparative example 4-1.
  • FIG. 16 is a diagram illustrating the flexural strength and Vickers hardness of test samples of example 5-1 and comparative example 5-1, and the results of observing cross sections of the test samples and raw material powders using a scanning electron microscope.
  • FIG. 17 is a diagram illustrating the flexural strength and Vickers hardness of test samples of examples 6-1 and 6-2, and the results of observing cross sections of the test samples using a scanning electron microscope.
  • FIG. 18 is a diagram illustrating the result of observing a cross section of a test sample of example 6-1 using a scanning electron microscope.
  • a boehmite structure 1 includes multiple boehmite particles 2 . Adjacent boehmite particles 2 are bonded to each other to form the boehmite structure 1 formed by combining boehmite particles 2 . Further, there are pores 3 among the adjacent boehmite particles 2 .
  • the boehmite particles 2 may be particles made from only a boehmite phase or may be particles made from a mixed phase of boehmite, and an aluminum oxide or an aluminum hydroxide other than boehmite.
  • the boehmite particles 2 may be particles in which a phase made from boehmite and a phase made from gibbsite (Al(OH) 3 ) are mixed.
  • the average particle size of the boehmite particles 2 making up the boehmite structure 1 is not particularly limited but is preferably within a range of 300 nm or more and 20 ⁇ m or less, more preferably of 300 nm or more and 10 ⁇ m or less, and particularly preferably of 300 nm or more and 5 ⁇ m or less.
  • a boehmite crystallite included in the boehmite structure 1 become small, which enhances the strength of the boehmite structure 1 .
  • the average particle size of the boehmite particles 2 is within these ranges, the percentage of pores in the boehmite structure 1 is 15% or less, as is described below, and thus it becomes possible to enhance the strength of the boehmite structure 1 .
  • the value of the “average particle size” is, unless otherwise stated, a value calculated as an average value of particle sizes of particles observed in several to several tens of visual fields by using observation means, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
  • the shape of the boehmite particles 2 is not particularly limited but may be spherical, for example.
  • the boehmite particles 2 may be particles having the shape of a whisker (needle) or particles having the shape of a scale.
  • the whisker-shaped particles or scale-shaped particles have a higher contact with other particles than the spherical particles, and thus it becomes possible to enhance the strength of the entire boehmite structure 1 .
  • the boehmite structure 1 is made from a particle group of boehmite particles 2 . That is, the boehmite structure 1 is made from multiple boehmite particles 2 mainly made from boehmite, and the boehmite structure 1 is formed by bonding the boehmite particles 2 to each other. In this case, the boehmite particles 2 may be in point contact with each other, or particle surfaces of the boehmite particles 2 may be in surface contact with each other.
  • adjacent boehmite particles 2 are bonded through at least one of an oxide or a hydroxide of aluminum. That is, the boehmite particles 2 are not bonded by an organic binder made from an organic compound, and are not bonded by an inorganic binder made from an inorganic compound other than an oxide and a hydroxide of aluminum. That is, as is described below, the boehmite structure 1 is formed by heating a mixture of mechanochemically treated hydraulic alumina and water under pressure. Thus, since the boehmite structure 1 does not include any impurities derived from a reaction promoter, it becomes possible to maintain properties inherent to boehmite.
  • the boehmite structure 1 has at least a boehmite phase made from boehmite (AlOOH) but may also have another crystalline phase other than the boehmite phase.
  • Examples of another crystalline phase other than the boehmite phase of the boehmite structure 1 include a gibbsite phase made from an aluminum hydroxide (Al(OH) 3 ), and a ⁇ -alumina phase made from an alumina oxide (Al 2 O 3 ).
  • the boehmite structure 1 is mainly made from the boehmite phase.
  • boehmite is lightweight and has higher chemical stability and heat resistance
  • mainly using the boehmite phase provides the boehmite structure 1 that is lightweight and excellent in chemical stability and heat resistance.
  • the percentage of presence of the boehmite phase is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. Increasing the percentage of the boehmite phase provides the boehmite structure 1 that is lightweight and excellent in chemical stability and heat resistance. Note that the percentage of the boehmite phase in the boehmite structure 1 is determined by measuring an X-ray diffraction pattern of the boehmite structure 1 using an X-ray diffraction method and then performing a Rietveld analysis.
  • the boehmite structure 1 may include a gibbsite phase made from an aluminum hydroxide (Al(OH) 3 ) in addition to the boehmite phase.
  • Al(OH) 3 aluminum hydroxide
  • the boehmite structure 1 is heated to have the gibbsite phase dehydrated. That is, by heating the boehmite structure 1 , a dehydration reaction occurs, which causes the crystalline structure of the gibbsite phase to change to that of the boehmite phase.
  • the heating condition of the boehmite structure 1 is not particularly limited as long as the dehydration reaction of the gibbsite phase occurs, but for example, preferably, the boehmite structure 1 is heated to 300° C. or higher in air.
  • the hydraulic alumina which is a raw material
  • the hydraulic alumina is heated to reduce the presence percentage of gibbsite in the hydraulic alumina.
  • the boehmite structure 1 is also formed by heating a mixture of the hydraulic alumina with reduced gibbsite and water under pressure as described below.
  • the boehmite crystallite size is 10 nm or less. As the crystallite size of boehmite included in the boehmite structure 1 becomes smaller, the flexural strength of the obtained boehmite structure 1 improves, and thus it becomes possible to enhance the strength of the boehmite structure 1 . Note that in the boehmite structure 1 , the boehmite crystallite size is more preferably 8 nm or less, and even more preferably 5 nm or less.
  • the crystallite size of boehmite included in the boehmite structure 1 can be obtained by measuring the diffraction peak of the boehmite with a powder X-ray diffraction method and then using Scherrer equation with the full width at half maximum and diffraction angle (Bragg angle of the diffraction X-ray) of the diffraction peak. Specifically, after the boehmite structure 1 is ground, the powder X-ray diffraction measurement is performed on the boehmite structure 1 . Then, the crystallite size can be obtained from the full width at half maximum and diffraction angle of the diffraction peak of the boehmite in the boehmite structure 1 by using Scherrer equation in equation 1.
  • D crystallite size (nm)
  • K Scherrer constant
  • ⁇ wavelength of the measured X-ray (nm)
  • ⁇ full width at half maximum (deg)
  • ⁇ Bragg angle of the X-ray diffraction (deg)
  • the porosity in the cross section of the boehmite structure 1 is preferably 15% or less. That is, when the cross section of the boehmite structure 1 is observed, the average value of the percentage of pores per unit area is preferably 15% or less. When the porosity is 15% or less, the bonding ratio of boehmite particles 2 increases, and thus the boehmite structure 1 becomes dense and has increased strength. Thus, it becomes possible to improve the durability of the boehmite structure 1 . When the porosity is 15% or less, the occurrence of cracks originating from the pores 3 in the boehmite structure 1 is prevented, and thus it becomes possible to increase the flexural strength of the boehmite structure 1 .
  • the porosity in the cross section of the boehmite structure 1 is preferably 10% or less, more preferably 5% or less, and even more preferably 3% or less. As the porosity in the cross section of the boehmite structure 1 becomes smaller, the crack originating from the pores 3 is more prevented, and thus it becomes possible to enhance the strength of the boehmite structure 1 .
  • the porosity is determined as follows. First, the cross section of the boehmite structure 1 is observed, and the boehmite particles 2 and the pores 3 are identified. Then, the unit area and the area of the pores 3 in the unit area are measured to obtain the percentage of the pores 3 per unit area. After the percentage of the pores 3 per unit area is obtained at multiple locations, the average value of the percentage of the pores 3 per unit area is used as the porosity. Note that when observing the cross section of the boehmite structure 1 , an optical microscope, a scanning electron microscope (SEM), or a transmission electron microscope (TEM) is usable. The unit area and the area of the pores 3 in the unit area may be measured by binarizing an image observed using the microscope.
  • SEM scanning electron microscope
  • TEM transmission electron microscope
  • the size of each of the pores 3 present inside the boehmite structure 1 is not particularly limited, but preferably, it is as small as possible.
  • the smaller size of the pore 3 prevents cracks originating from the pore 3 , and thus it becomes possible to increase the strength of the boehmite structure 1 and improve durability of the boehmite structure 1 .
  • the size of the pore 3 in the boehmite structure 1 is preferably 5 ⁇ m or less, more preferably 1 ⁇ m or less, and even more preferably 100 nm or less.
  • the size of the pore 3 present inside the boehmite structure 1 is determined by observing the cross section of the boehmite structure 1 using a microscope in the same manner as the porosity described above.
  • the boehmite structure 1 only needs to have a structure in which the boehmite particles 2 are bonded to each other, the crystallite size of the boehmite is 10 nm or less, and the porosity is 15% or less.
  • the boehmite structure 1 has such a structure, its shape is not particularly limited.
  • the boehmite structure 1 may have the shape of, for example, a plate, a film, a rectangle, a block, a rod, or a sphere.
  • a thickness t is not particularly limited but can be, for example, 50 ⁇ m or more.
  • the boehmite structure 1 according to the present embodiment is formed using a pressure heating method as described below. Thus, the boehmite structure 1 having a large thickness is easily obtained.
  • the thickness t of the boehmite structure 1 may be 1 mm or more or may be 1 cm or more.
  • the upper limit of the thickness t of the boehmite structure 1 is not particularly limited but may be, for example, 50 cm.
  • the boehmite crystallite size and porosity are within predetermined ranges, and thus the boehmite structure 1 has a high mechanical strength.
  • the boehmite structure 1 preferably has a flexural strength of 50 MPa or more measured in accordance with Japanese Industrial Standard JIS T6526: 2018 (Dental ceramic materials). Note that the flexural strength of the boehmite structure 1 is measured using the biaxial flexure test of JIS T6526. When the flexural strength of the boehmite structure 1 is 50 MPa or more, the boehmite structure 1 is excellent in mechanical strength, and the machinability is enhanced.
  • the flexural strength of the boehmite structure 1 is preferably 80 MPa or more, and more preferably 100 MPa or more.
  • the upper limit of the flexural strength of the boehmite structure 1 is not particularly limited but may be, for example, 300 MPa.
  • multiple boehmite particles 2 are not bonded using an organic binder made from an organic compound and are further not bonded using an inorganic binder made from an inorganic compound other than an oxide and a hydroxide of aluminum.
  • the content ratio of the elements other than aluminum is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. Since the boehmite structure 1 hardly includes impurities, such as sodium or calcium, it becomes possibel to maintain the properties inherent to boehmite.
  • the boehmite structure 1 includes multiple boehmite particles 2 where adjacent boehmite particles 2 are bonded to each other.
  • the boehmite structure 1 has a boehmite crystallite size of 10 nm or less and a porosity of 15% or less.
  • the boehmite structure 1 according to the present embodiment has a boehmite crystallite size of 10 nm or less and a porosity of 15% or less, and thus the boehmite particles 2 are densely and tightly bonded to each other. Consequently, the boehmite structure 1 has improved mechanical strength and thus can have high durability.
  • boehmite structure 1 multiple boehmite particles 2 are bonded without using an organic binder and an inorganic binder made from an inorganic compound other than an oxide and a hydroxide of aluminum.
  • the boehmite structure 1 mainly includes a boehmite phase.
  • the boehmite structure 1 is lightweight and has excellent chemical stability.
  • the boehmite structure 1 can be obtained by mixing mechanochemically treated hydraulic alumina and a solvent including water, and then heating the mixture under pressure.
  • the hydraulic alumina is a hydrate formed by heat treatment of aluminum hydroxide and includes p-alumina. Such hydraulic alumina has a property of bonding and curing through a hydration reaction.
  • the pressure heating method the hydration reaction of the hydraulic alumina progresses to bond particles of the hydraulic alumina to each other, and the crystalline structure changes to that of boehmite, so that the boehmite structure 1 can be obtained.
  • the powder of hydraulic alumina is subjected to a mechanochemical treatment, and coarse particles of the hydraulic alumina are ground into fine particles.
  • the specific surface area of the hydraulic alumina increases, and the hydration reaction between the hydraulic alumina and a solvent including water is promoted.
  • the mechanochemical treatment on the hydraulic alumina hydroxy groups on the surface of the hydraulic alumina particles increase, and the reactivity with the solvent increases. Consequently, the hydraulic alumina particles are easily bonded to each other, and thus it becomes possible to obtain the boehmite structure 1 having high strength.
  • the mechanochemical treatment of the hydraulic alumina powder is not particularly limited as long as the coarse particles of the hydraulic alumina are ground into fine particles.
  • the mechanochemical treatment is preferably a grinding treatment using at least one selected from the group consisting of a ball mill, a bead mill, or a vibration mill.
  • the mechanochemical treatment is preferably a grinding treatment using a planetary ball mill.
  • the planetary ball mill is a ball mill that grinds hydraulic alumina through the rotational movement of a pot including hydraulic alumina and hard balls (media) and the rotation (orbital movement) of a stage on which the pot is placed.
  • such a planetary ball mill applies a stronger centrifugal force by rotating and revolving in opposite directions, and thus the grinding treatment at a submicron level can be performed in a short time.
  • the hard balls (media) used in the mechanochemical treatment are not particularly limited, and hard balls made from ceramic, a resin, or a metal can be used.
  • the pot used in the mechanochemical treatment is also not particularly limited, and a pot made from ceramic, a resin, or a metal can be used. Ceramic hard balls and pot can be made from zirconia, alumina, agate, or silicon nitride. Note that from the viewpoint of efficiently grinding hydraulic alumina, the hard balls and pot are preferably made from ceramic.
  • the mechanochemical treatment of the hydraulic alumina powder may be a wet treatment, a dry treatment, or a combination of a wet treatment and a dry treatment.
  • the mechanochemical treatment is preferably a wet treatment.
  • an organic solvent is preferably used as a solvent for wet grinding.
  • an alcohol is preferably used as a solvent for wet grinding, and ethanol is more preferably used. Since ethanol is difficult to react with hydraulic alumina and can be easily removed after grinding, it can be suitably used as a solvent for wet grinding.
  • the average particle size of the hydraulic alumina powder subjected to the mechanochemical treatment is not particularly limited but is preferably within a range of 0.1 ⁇ m to 5 ⁇ m, and more preferably of 0.5 ⁇ m to 3 ⁇ m.
  • the particle size of the hydraulic alumina is within these ranges, the hydration reaction between the hydraulic alumina and a solvent including water proceeds more easily, and thus bonding between the hydraulic alumina particles is promoted. Since the crystallite size of boehmite in the boehmite structure 1 is 10 nm or less, it becomes possible to further improve the strength of the boehmite structure 1 .
  • the average particle size of the hydraulic alumina powder can be measured using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
  • a mixture is prepared by mixing a powder of mechanochemically treated hydraulic alumina with a solvent including water.
  • the solvent including water is pure water or ion-exchanged water.
  • the solvent including water may include an acidic substance or an alkaline substance other than water.
  • the solvent including water is only required to be made mainly from water and may include, for example, an organic solvent (for example, alcohol).
  • the amount of the solvent added to the hydraulic alumina is an amount sufficient to proceed the hydration reaction of the hydraulic alumina.
  • the amount of the solvent added is preferably 20% to 200% by mass, and more preferably 50% to 150% by mass, relative to the hydraulic alumina.
  • the mixture formed by mixing the hydraulic alumina with the solvent including water is filled in a mold.
  • the mold may be heated as necessary.
  • pressure to the mixture inside the mold the inside of the mold becomes a high pressure state.
  • the hydraulic alumina becomes highly filled, and the particles of the hydraulic alumina are bonded to each other, resulting in a high density.
  • the hydraulic alumina undergoes a hydration reaction to form boehmite and aluminum hydroxide on the surface of the hydraulic alumina particles.
  • the generated boehmite and aluminum hydroxide both diffuse between adjacent hydraulic alumina particles, and the hydraulic alumina particles are gradually bonded to each other. Then, the dehydration reaction proceeds by heating, and the crystalline structure changes from aluminum hydroxide to boehmite. Note that it is supposed that the above-described hydration reaction of hydraulic alumina, interdiffusion between hydraulic alumina particles, and dehydration reaction proceed almost simultaneously.
  • the molded body is then taken out from the mold to obtain the boehmite structure 1 in which multiple boehmite particles 2 are bonded to each other via at least one of the oxide or the hydroxide of aluminum.
  • the condition for heating and pressurizing the mixture formed by mixing the hydraulic alumina with the solvent including water is not particularly limited as long as the reaction between the hydraulic alumina and the solvent progresses.
  • the mixture formed by mixing the hydraulic alumina and the solvent including water is pressurized at a pressure of 10 to 600 MPa while being heated to 50 to 300° C.
  • the temperature at which the mixture of the hydraulic alumina and the solvent including water is heated is more preferably within a range of 80 to 250° C., and even more preferably of 100 to 200° C.
  • the pressure at which the mixture formed by mixing the hydraulic alumina and the solvent including water is pressurized is more preferably within a range of 50 to 600 MPa, and even more preferably of 200 to 600 MPa.
  • the boehmite structure As a method for forming the boehmite structure, a method for pressing only the powder of boehmite is considered. However, even if the powder of boehmite is put into a mold and pressurized at normal temperature, the particles of boehmite are difficult to react with each other, and it is difficult to firmly bond the particles together. Thus, the obtained compact has many pores and thus has insufficient mechanical strength.
  • the sintering method is a method for obtaining a sintered body by heating an aggregate of solid powders made from an inorganic substance at a temperature lower than the melting point.
  • a method for forming the boehmite structure a method for pressing only the powder of boehmite to form a compact and then calcining it at 500° C. is also considered.
  • the compact is calcined at 500° C., the dehydration reaction of boehmite progresses, and the crystalline structure changes from boehmite to ⁇ -alumina.
  • a further method for forming the boehmite structure is considered to include forming a compact by pressing only the powder of boehmite and then calcining it at 1400° C.
  • the compact of boehmite powder is calcined at 1400° C.
  • the boehmite powders are sintered to form a structure.
  • the compact of boehmite is calcined at 1400° C.
  • the dehydration reaction of boehmite progresses, and the crystalline structure changes from boehmite to a-alumina.
  • the method for producing the boehmite structure 1 according to the present embodiment includes a mixing step of obtaining a mixture by mixing mechanochemically treated hydraulic alumina with a solvent including water, and a pressure heating step of pressurizing and heating the mixture.
  • the condition for heating and pressurizing the mixture is a temperature of 50 to 300° C., and a pressure of 10 to 600 MPa.
  • the boehmite structure 1 is formed under such a low temperature condition, the obtained structure is mainly made from a boehmite phase.
  • the boehmite structure 1 that is lightweight, excellent in chemical stability, and has a reduced amount of impurities can be obtained using a simple method.
  • the producing method according to the present embodiment uses mechanochemically treated hydraulic alumina, the crystallite size of boehmite in the boehmite structure 1 can be reduced to 10 nm or less.
  • the mechanochemically treated hydraulic alumina By using the mechanochemically treated hydraulic alumina, the hydration reaction between the hydraulic alumina and a solvent including water is promoted, and the particles of the hydraulic alumina are easily bonded to each other. Therefore, the boehmite structure 1 having high strength can be obtained.
  • a boehmite structure 1 A according to the second embodiment includes multiple boehmite particles 2 , and adjacent boehmite particles 2 are bonded to each other as in the first embodiment.
  • the boehmite structure 1 A further includes inorganic oxide particles 4 in addition to the boehmite particles 2 , and the inorganic oxide particles 4 are highly dispersed in the boehmite structure 1 A. That is, the boehmite structure 1 A according to the present embodiment is a molded body that includes multiple boehmite particles 2 and multiple inorganic oxide particles 4 and is formed by bonding at least the boehmite particles 2 to each other. Since the boehmite structure 1 A includes inorganic oxide particles 4 , the inorganic oxide particles 4 act as an aggregate, and thus it becomes possible to enhance the strength of the boehmite structure 1 A.
  • the boehmite structure 1A preferably has a boehmite crystallite size of 10 nm or less, more preferably 8 nm or less, and even more preferably 5 nm or less. As in the first embodiment, as the crystallite size of the boehmite included in the boehmite structure 1 A becomes smaller, it becomes possible to enhance the strength of the obtained boehmite structure 1 A.
  • the boehmite structure 1 A includes the inorganic oxide particles 4 in addition to the boehmite particles 2 .
  • the inorganic oxide particles 4 often have multiple hydroxy groups on the surface of the particles.
  • the boehmite particles 2 and inorganic oxide particles 4 are easily bonded through dehydration condensation. Consequently, since the boehmite particles 2 and inorganic oxide particles 4 are firmly bonded, it becomes possible to enhance the strength of the boehmite structure 1 A.
  • the inorganic oxide particles 4 are made from an oxide of at least one metallic element selected from the group consisting of an alkali metal, an alkaline earth metal, a transition metal, a base metal, and a semimetal.
  • the alkaline earth metal includes beryllium and magnesium in addition to calcium, strontium, barium, and radium.
  • the base metal includes aluminum, zinc, gallium, cadmium, indium, tin, mercury, thallium, lead, bismuth, and polonium.
  • the semimetal includes boron, silicon, germanium, arsenic, antimony, and tellurium.
  • particles can be used that include at least one selected from the group consisting of alumina (Al 2 O 3 ), zirconia (ZrO 2 ), mullite (3Al 2 O 3 ⁇ 2SiO 2 ), zircon (ZrSiO 4 ), cordierite (2MgO ⁇ 2Al 2 O 3 ⁇ 5SiO 2 ), forsterite (2MgO ⁇ SiO 2 ), yttria (Y 2 O 3 ), steatite (MgO ⁇ SiO 2 ), silica (quartz glass, SiO 2 ), titanium oxide (TiO 2 ), copper oxide (CuO), and iron oxide (Fe 2 O 3 ).
  • alumina Al 2 O 3
  • ZrO 2 zirconia
  • mullite 3Al 2 O 3 ⁇ 2SiO 2
  • zircon zircon
  • cordierite 2MgO ⁇ 2Al 2 O 3 ⁇ 5SiO 2
  • forsterite 2MgO ⁇ SiO 2
  • the boehmite structure 1 A may include inorganic carbide particles instead of the inorganic oxide particles 4 .
  • the boehmite structure 1 A may include inorganic carbide particles in addition to the inorganic oxide particles 4 . Since inorganic carbide particles also act as an aggregate, it becomes possible to enhance the strength of the boehmite structure 1 A by including the inorganic carbide particles.
  • the inorganic carbide particles particles made from a carbide of at least one metallic element selected from the group consisting of an alkali metal, alkaline earth metal, transition metal, base metal, and metalloid can be used.
  • particles made from silicon carbide (SiC) can also be used.
  • the inorganic oxide particles 4 preferably include silicon.
  • the inorganic oxide particles 4 preferably include silica (SiO 2 ).
  • silica SiO 2
  • hydroxy groups are easily formed on the surface of the particles. Consequently, the boehmite particles 2 and the inorganic oxide particles 4 are easily bonded through dehydration condensation, and thus it becomes possible to further enhance the strength of the boehmite structure 1 A.
  • the inorganic oxide particles including silica are preferably particles made from at least one selected from the group consisting of mullite (3Al 2 O 3 ⁇ 2SiO 2 ), zircon (ZrSiO 4 ), cordierite (2MgO ⁇ 2Al 2 O 3 ⁇ 5SiO 2 ), forsterite (2MgO ⁇ SiO 2 ), steatite (MgO ⁇ SiO 2 ), and quartz glass (SiO 2 ).
  • the average particle size of the inorganic oxide particles 4 is preferably 5 ⁇ m or less.
  • the smaller the particle size of the inorganic oxide particles 4 the greater the specific surface area, which can increase the contact parts between the boehmite particles 2 and the inorganic oxide particles 4 and thus can increase the adhering parts. Consequently, it becomes possible to enhance the strength of the boehmite structure 1 A.
  • the average particle size of the inorganic oxide particles 4 is more preferably 3 ⁇ m or less, and even more preferably 1 ⁇ m or less.
  • the average particle size of the inorganic oxide particles 4 can be measured using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
  • the average particle size of the inorganic oxide particles 4 may be within a range of 0.1 mm to 10 cm. It becomes possible to enhance the hardness of the boehmite structure 1 A by adding, to the boehmite structure 1 A, inorganic oxide particles 4 having a relatively large particle size. That is, by including the inorganic oxide particles 4 having a high hardness and a large particle size, it becomes possible, due to the inorganic oxide particles 4 , to enhance the hardness of the boehmite structure 1 A itself.
  • the boehmite structure 1 A having high strength can be obtained through the fixation of the hydraulic alumina and inorganic oxide particles which are in closest packing.
  • a value is adopted that is measured for the material making up the inorganic oxide particles 4 according to JIS R1610 (Test methods for hardness of fine ceramics).
  • the inorganic oxide particles 4 preferably have an elastic modulus (Young's modulus) of 320 GPa or less. If the elastic modulus of the inorganic oxide particles 4 is small, when the mixture of the hydraulic alumina particles, inorganic oxide particles 4 , and solvent is pressurized, the inorganic oxide particles 4 can bend in the mixture. Thus, when the pressure reduction operation is performed after the hydraulic alumina and the inorganic oxide particles 4 are fixed through the heating and pressurization process to form the boehmite structure 1 A, the inorganic oxide particles can maintain the original shape inside the boehmite structure 1 A. Consequently, peeling at the interface between particles can be prevented, and thus it becomes possible to maintain the high strength of the boehmite structure 1 A.
  • an elastic modulus Young's modulus
  • the elastic modulus (Young's modulus) of the inorganic oxide particles 4 a value is adopted that is measured for the material making up the inorganic oxide particles 4 according to JIS R1602 (Testing methods for elastic modulus of fine ceramics).
  • Table 1 illustrates representative values of Vickers hardness of alumina, zirconia, mullite, zircon, cordierite, forsterite, and silica (quartz glass) which can be constituent materials of the inorganic oxide particles 4 .
  • Table 1 also illustrates representative values of the flexural strength, coefficient of thermal expansion, and specific gravity of these inorganic oxides.
  • Table 1 also illustrates representative values of the Vickers hardness, flexural strength, coefficient of thermal expansion, and specific gravity of boehmite.
  • Table 2 illustrates representative values of elastic moduli (Young's moduli) of alumina, mullite, zirconia, yttria, forsterite, cordierite, steatite, and silica.
  • the boehmite phase is preferably an indefinite shape crystal. That is, when the cross section of the boehmite structure 1 A is observed using a microscope, it is preferable that the boehmite phase have many irregularities on the surface and the shape of the boehmite phase be not constant. In addition, the boehmite phase preferably does not have a faceted surface, that is, a flat surface on the surface. Since the boehmite phase is an indefinite shape crystal, the contact area between the boehmite particles 2 (boehmite phase) and the inorganic oxide particles 4 increases, and thus the bonding force therebetween can be enhanced.
  • the inorganic oxide particles 4 may have a particle shape and may further have a faceted surface, that is, a flat surface, on the particle surface.
  • the inorganic oxide particles 4 are spherical particles having faceted surfaces, the dispersibility of the inorganic oxide particles 4 is enhanced during molding, and thus the inorganic oxide particles 4 can be dispersed almost uniformly in the boehmite structure 1 A. Consequently, it becomes possible to prevent the discrepancies of the strength and hardness of the boehmite structure 1 A.
  • the inorganic oxide particles 4 have faceted surfaces, there is a possibility that a pinning effect to suppress crystal growth of hydraulic alumina is exerted during molding of the boehmite structure 1 A. Consequently, it becomes possible to prevent the cracking of the boehmite particles 2 and further enhance the strength of the boehmite structure 1 A.
  • the inorganic oxide particles 4 may have a particle shape and may further have an indefinite shape without faceted surfaces.
  • the inorganic oxide particles 4 are indefinite shape particles, points of entanglement with the indefinite shape boehmite particles 2 increase. Consequently, the shear strength between the inorganic oxide particles 4 and the boehmite particles 2 increases, and thus it becomes possible to further improve the strength of the entire boehmite structure 1 A.
  • the content ratio of the boehmite particles 2 and the inorganic oxide particles 4 is not particularly limited. That is, in the boehmite structure 1 A, when the characteristics of boehmite is to be increased, it is necessary to decrease the content ratio of the inorganic oxide particles 4 . For example, when the strength of the boehmite structure 1 A is to be further increased, the content ratio of the inorganic oxide particles 4 may be increased.
  • the volume percentage of the inorganic oxide particles 4 can be within a range of 10% to 90% by volume, of 20% to 70% by volume, or of 25% to 60% by volume.
  • the boehmite structure 1 A can have, for example, a plate shape, film shape, rectangular shape, block shape, rod shape, or spherical shape.
  • thickness t of the boehmite structure 1 A is not particularly limited but can be, for example, 50 ⁇ m or more.
  • the thickness t of the boehmite structure 1 A may be 1 mm or more, or may be 1 cm or more.
  • the upper limit of the thickness t of the boehmite structure 1 A is not particularly limited but can be, for example, 50 cm.
  • the boehmite structure 1 A has a high mechanical strength.
  • the boehmite structure 1 A also preferably has a flexural strength of 50 MPa or more measured in accordance with JIS T6526:2018.
  • the flexural strength of the boehmite structure 1 A is more preferably 80 MPa or more, and even more preferably 100 MPa or more.
  • the upper limit of the flexural strength of the boehmite structure 1 A is not particularly limited, it can be, for example, 300 MPa.
  • the boehmite structure 1 A includes multiple boehmite particles 2 and multiple inorganic oxide particles 4 , and adjacent boehmite particles 2 are bonded to each other.
  • the boehmite structure 1 A has a boehmite crystallite size of 10 nm or less and a porosity of 15% or less.
  • the boehmite crystallite size is 10 nm or less and the porosity is 15% or less, and thus the boehmite particles 2 and the inorganic oxide particles 4 are densely and tightly fixed. Consequently, the boehmite structure 1 A has improved mechanical strength and thus can have high durability.
  • the inorganic oxide particles 4 as an aggregate are dispersed, and thus it becomes possible to enhance the strength.
  • the boehmite structure 1 A can be obtained by using a pressure heating method as in the first embodiment. That is, the boehmite structure 1 A can be obtained by mixing mechanochemically treated hydraulic alumina and inorganic oxide particles with a solvent including water, and then heating the mixture under pressure. By using such a pressure heating method, the hydration reaction of hydraulic alumina proceeds in a state where the hydraulic alumina and inorganic oxide particles are mixed. Then, while the hydraulic alumina bonds with each other, the crystalline structure changes to boehmite. Consequently, the boehmite structure 1 A can be obtained which includes the boehmite particles 2 and the inorganic oxide particles 4 and in which adjacent boehmite particles 2 are bonded to each other.
  • the powder of hydraulic alumina is subjected to a mechanochemical treatment.
  • the mechanochemical treatment of the hydraulic alumina can be performed in the same manner as in the first embodiment.
  • the average particle size of the hydraulic alumina powder subjected to the mechanochemical treatment is not particularly limited, it is preferably within a range of 0.1 ⁇ m to 5 ⁇ m, and more preferably of 0.5 ⁇ m to 3 ⁇ m.
  • the mechanochemical treatment of the inorganic oxide particles can be performed in the same manner as the mechanochemical treatment of the hydraulic alumina. That is, the mechanochemical treatment is preferably a grinding treatment using at least one selected from the group consisting of a ball mill, a bead mill, or a vibration mill. Among them, the mechanochemical treatment is preferably a grinding treatment using a planetary ball mill.
  • the mechanochemical treatment of the powder of inorganic oxide particles may be a wet treatment, a dry treatment, or a combination of a wet treatment and a dry treatment. Note that when the inorganic oxide particles are difficult to react with water, water may be used as a solvent for wet grinding.
  • the average particle size of the powder of inorganic oxide particles subjected to the mechanochemical treatment is not particularly limited, it is preferably within a range of 0.1 ⁇ m to 5 ⁇ m, and more preferably of 0.5 ⁇ m to 3 ⁇ m. Since the average particle sizes of the inorganic oxide particles and hydraulic alumina subjected to the mechanochemical treatment are equivalent, the inorganic oxide particles and the hydraulic alumina are easily mixed uniformly. Thus, it becomes possible to promote adhesion between the inorganic oxide particles and the hydraulic alumina and enhance the strength of the boehmite structure 1 A. Note that the average particle size of the powder of the inorganic oxide particles can be measured using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
  • SEM scanning electron microscope
  • TEM transmission electron microscope
  • the mechanochemical treatment of the hydraulic alumina and the mechanochemical treatment of the inorganic oxide particles may be performed separately or together. That is, after the mechanochemical treatment is performed separately on the hydraulic alumina and the inorganic oxide particles, they may be mixed. Further, after the hydraulic alumina and inorganic oxide particles are put into one pot, they may be subjected to the mechanochemical treatment together. However, since it is preferable to mix the hydraulic alumina and inorganic oxide particles uniformly, it is preferable to perform the mechanochemical treatment on them together.
  • the solvent including water is preferably pure water or ion-exchanged water.
  • the solvent including water may include an acidic substance or an alkaline substance, or may include an organic solvent.
  • the amount of the solvent added to the mixture of the hydraulic alumina and inorganic oxide particles is preferably an amount sufficient to proceed the hydration reaction of the hydraulic alumina.
  • the amount of the solvent added is, relative to the hydraulic alumina, preferably within a range of 20% to 200% by mass, and more preferably of 50% to 150% by mass.
  • the mixture obtained by mixing the hydraulic alumina and inorganic oxide particles with the solvent including water is filled into a mold.
  • the mold may be heated as necessary.
  • the inside of the mold becomes a high pressure state.
  • the hydraulic alumina and inorganic oxide particles become highly filled, and the hydraulic alumina particles are bonded to each other.
  • the dehydration reaction proceeds through heating, and the boehmite particles 2 are bonded to each other.
  • the dehydration condensation reaction with the boehmite particles 2 proceeds, and thus the boehmite particles 2 and the inorganic oxide particles 4 are bonded to each other.
  • the molded body is then taken out from the mold to obtain the boehmite structure 1 A in which the boehmite particles 2 are bonded to each other and further the inorganic oxide particles 4 are dispersed.
  • the condition for heating and pressurizing the mixture obtained by mixing the hydraulic alumina and inorganic oxide particles with the solvent including water are preferably pressurizing at a pressure of 10 to 600 MPa while heating to 50 to 300° C., as in the first embodiment.
  • the temperature at which the mixture is heated is more preferably within a range of 80 to 250° C. and more preferably of 100 to 200° C.
  • the pressure at which the mixture is pressurized is more preferably within a range of 50 to 600 MPa and more preferably of 200 to 600 MPa.
  • the producing method of the boehmite structure 1 A includes a mixing step of obtaining a mixture by mixing mechanochemically treated hydraulic alumina and inorganic oxide particles with a solvent including water, and a pressure heating step of pressurizing and heating the mixture.
  • the condition for heating and pressurizing the mixture is preferably a temperature of 50 to 300° C. and a pressure of 10 to 600 MPa.
  • the mechanochemically treated hydraulic alumina and inorganic oxide particles are used.
  • the crystallite size of boehmite in the boehmite structure 1 A is set to 10 nm or less, in addition, the inorganic oxide particles acting as an aggregate can be highly dispersed. Therefore, the boehmite structure 1 A having high strength can be obtained.
  • hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared as hydraulic alumina. Note that the hydraulic alumina has a central particle size of 16 ⁇ m.
  • the hydraulic alumina powder was analyzed using a powder X-ray diffraction method and was found to be a mixture of boehmite and gibbsite (aluminum hydroxide).
  • the hydraulic alumina also included ⁇ alumina.
  • the hydraulic alumina was subjected to the mechanochemical treatment using a planetary ball mill. Specifically, first, the hydraulic alumina, ethanol as a solvent, and hard balls (media) were put into a resin pot. Note that the pot used was made from polyamide and had a volume of 250 mL. The media used were made from zirconia and had a diameter of ⁇ 3 mm.
  • the hydraulic alumina was ground by treating, with a planetary ball mill, the pot including the hydraulic alumina, solvent, and hard balls.
  • the planetary ball mill used was a planetary ball-type mill, Classic Line P-5, manufactured by Fritsch Japan Co., Ltd.
  • the rotation speed of the pot was set to 200 rpm, and the treatment time was set to 3 hours.
  • the mixture of the hydraulic alumina and solvent was taken out from the pot, and the solvent was removed to obtain mechanochemically treated hydraulic alumina.
  • hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared. After ion-exchanged water was weighed to be 80% by mass relative to the same hydraulic alumina, the hydraulic alumina and the ion-exchanged water were mixed using an agate mortar and pestle to obtain a mixture. Next, the mixture obtained was put into a cylindrical molding die ( ⁇ 10) having an internal space. The mixture was heated and pressurized under a condition of 400 MPa, 180° C., and 20 minutes to obtain a test sample of comparative example 1-1. That is, the test sample of comparative example 1-1 is a sample in which no mechanochemical treatment has been performed on the hydraulic alumina.
  • Flexural strength was measured in accordance with JIS T6526 for the test samples of example 1-1 and comparative example 1-1. Consequently, as illustrated in FIG. 3 , the flexural strength (the maximum value of stress) of the test sample of example 1-1 was 83.2 MPa. In contrast, the flexural strength of the test sample of comparative example 1-1 was 51.4 MPa.
  • Vickers hardness was measured in accordance with JIS R1610 for the test samples of example 1-1 and comparative example 1-1. As illustrated in FIG. 3 , the Vickers hardness of the test sample of example 1-1 was 2.48 GPa. In contrast, the Vickers hardness of the test sample of comparative example 1-1 was 1.87 GPa.
  • hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared as hydraulic alumina.
  • the hydraulic alumina was subjected to mechanochemical treatment using a planetary ball mill. Specifically, first, the hydraulic alumina, ethanol as a solvent, and hard balls (media) were put into a resin pot. Note that the pot used was made from polyamide and had a volume of 250 mL. The media used were made from zirconia and had a diameter of ⁇ 3 mm.
  • the hydraulic alumina was ground by treating, with a planetary ball mill, the pot including the hydraulic alumina, solvent, and hard balls.
  • the planetary ball mill was the same as that of example 1-1.
  • the rotation speed of the pot was set to 200 rpm, and the treatment time was set to 1 hour.
  • the mixture of the hydraulic alumina and solvent was taken out from the pot, and the solvent was removed to obtain mechanochemically treated hydraulic alumina.
  • test sample of example 2-2 was obtained in the same manner as in example 2-1 except that a zirconia pot was used as the pot and the treatment time was set to 3 hours.
  • test sample prepared in the same manner as in comparative example 1-1 was used as the test sample of comparative example 2-1.
  • Powder X-ray diffraction measurements were performed on the test samples of examples 2-1 and 2-2 and comparative example 2-1 using an X-ray diffraction apparatus to determine the crystallite size. Specifically, powder X-ray diffraction measurements using Cu-K ⁇ radiation were performed after each test sample was ground. Then, the crystallite size was determined from the full width at half maximum and diffraction angle of the diffraction peak of boehmite in each test sample by using Scherrer equation in equation 1.
  • FIG. 4 illustrates X-ray diffraction patterns of test samples.
  • FIG. 4 also illustrates X-ray diffraction patterns of boehmite (AlOOH) and nordstrand (Al(OH) 3 ) registered in ICSD.
  • the hydraulic alumina was ground by treating, with a planetary ball mill, the pot including the hydraulic alumina, solvent, and hard balls.
  • the planetary ball mill was the same as that of example 1-1.
  • the rotation speed of the pot was set to 250 rpm, and the treatment time was set to 3 hours.
  • the mixture of the hydraulic alumina and solvent was taken out from the pot, and the solvent was removed to obtain the mechanochemically treated hydraulic alumina.
  • test sample of example 3-3 was obtained in the same manner as in example 3-1 except that a polyamide pot was used as the pot, the rotation speed of the pot was set to 200 rpm, and the treatment time was set to 1 hour.
  • Table 3 collectively illustrates pot materials, pot rotation speeds in planetary ball mills, and treatment times of the planetary ball mills in examples 3-1 to 3-3.
  • a cross section polisher processing (CP processing) was applied to a cross section of a test sample of example 3-1, which is cylindrical.
  • SEM scanning electron microscope
  • a reflected electron image was observed at a magnification of 2,000 times on the cross section of the test sample.
  • the reflected electron images obtained by observing three points (positions 1 to 3) in the cross section of the test sample are illustrated in FIGS. 5 ( a ), 6 ( a ), and 7 ( a ) .
  • gray particles are boehmite particles 2 and white portions are pores 3 .
  • the porosities of the test samples of example 3-2 and example 3-3 were determined in the same manner as that of the test sample of example 3-1.
  • the reflected electron images obtained by observing three points (positions 1 to 3) in the cross section of the test sample of example 3-2 are illustrated in FIGS. 8 ( a ), 9 ( a ), and 10 ( a ) .
  • the binarized images of the reflected electron images of FIGS. 8 ( a ), 9 ( a ), and 10 ( a ) are illustrated in FIGS. 8 ( b ), 9 ( b ), and 10 ( b ) , respectively.
  • FIGS. 11 ( a ), 12 ( a ), and 13 ( a ) The reflected electron images obtained by observing three points (positions 1 to 3) in the cross section of the test sample of example 3-3 are illustrated in FIGS. 11 ( a ), 12 ( a ), and 13 ( a ) . Further, the binarized images of the reflected electron images of FIGS. 11 ( a ), 12 ( a ), and 13 ( a ) are illustrated in FIGS. 11 ( b ), 12 ( b ), and 13 ( b ) . Table 3 collectively illustrates porosities of the test samples of example 3-1, example 3-2, and example 3-3.
  • the porosity of the test sample made from mechanochemically treated hydraulic alumina is less than 1%.
  • a boehmite structure having a boehmite crystallite size of 10 nm or less and a porosity of 15% or less has a high flexural strength.
  • the mechanochemically treated hydraulic alumina prepared in example 1-1 was used as the hydraulic alumina of this example.
  • Hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was used as the hydraulic alumina of this example.
  • the temperature of the mixture when water was mixed with hydraulic alumina in each example and the temperature of the water were measured, and the temperature change due to the reaction between the hydraulic alumina and water was evaluated. Specifically, 80% by mass ion-exchanged water was mixed with each hydraulic alumina in example 4-1 and comparative example 4-1, and the temperature of the mixtures and the temperature of the ion-exchanged water were measured. Table 4 illustrates the measurement results.
  • Infrared spectroscopic analysis was performed on the hydraulic alumina of example 4-1 and comparative example 4-1. Specifically, the surface infrared absorption spectrum of the hydraulic alumina of each example was measured using a total reflectance measurement (ATR method). FIG. 15 illustrates the measurement results.
  • hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared.
  • zircon (ZrSiO 4 ) zirconium silicate MZ-50B manufactured by DAIICHI KIGENSO KAGAKU KOGYO CO., LTD. was prepared.
  • the hydraulic alumina and zircon were each weighed to be 50% by volume, and the hydraulic alumina and zircon were subjected to mechanochemical treatment using a planetary ball mill. Specifically, first the hydraulic alumina, zircon, ethanol as a solvent, and hard balls (media) were put into a resin pot. Note that the pot used was made from polyamide and had a volume of 250 mL. The media used were made from zirconia and had a diameter of ⁇ 3 mm.
  • the hydraulic alumina and zircon were ground by treating, with a planetary ball mill, the pot including the hydraulic alumina, zircon, solvent, and hard balls.
  • the planetary ball mill was the same as that of example 1-1.
  • the rotation speed of the pot was set to 200 rpm, and the treatment time was set to 3 hours.
  • the mixture of the hydraulic alumina, zircon, and solvent was taken out from the pot, and the solvent was removed to obtain a mixture of mechanochemically treated hydraulic alumina and zircon.
  • the mixture of hydraulic alumina and zircon was mixed with the ion-exchanged water using an agate mortar and pestle to obtain a mixture.
  • the mixture obtained was put into a cylindrical mold ( ⁇ 10) having an internal space. The mixture was then heated and pressurized under a condition of 400 MPa, 180° C., and 20 minutes to obtain a test sample of example 5-1.
  • the same hydraulic alumina and zircon as in example 5-1 were prepared, and then the hydraulic alumina and zircon were each weighed to be 50% by volume.
  • the hydraulic alumina, zircon, and ion-exchanged water were mixed using an agate mortar and pestle to obtain a mixture.
  • the mixture obtained was put into a cylindrical mold ( ⁇ 10) having an internal space. The mixture was then heated and pressurized under a condition of 400 MPa, 180° C., and 20 minutes to obtain a test sample of comparative example 5-1. That is, the test sample of comparative example 5-1 is a sample in which hydraulic alumina and zircon are not subjected to mechanochemical treatment.
  • Flexural strength was measured in accordance with JIS T6526 for the test samples of example 5-1 and comparative example 5-1. Consequently, as illustrated in FIG. 16 , the flexural strength of the test sample of example 5-1 was 102.1 MPa. In contrast, the flexural strength of the test sample of comparative example 5-1 was 34.1 MPa.
  • Vickers hardness was measured in accordance with JIS R1610 for the test samples of example 5-1 and comparative example 5-1. As illustrated in FIG. 16 , the Vickers hardness of the test sample of example 5-1 was 3.0 GPa. In contrast, the Vickers hardness of the test sample of comparative example 5-1 was 1.8 GPa.
  • FIG. 16 also illustrates scanning electron micrographs of mixtures of hydraulic alumina and zircon prior to pressure heating treatment. It is evident from FIG. 16 that the mixture of hydraulic alumina and zircon in comparative example 5-1 includes a large number of coarse particles. In contrast, it is evident that the mixture of hydraulic alumina and zircon in example 5-1 does not include coarse particles as a result of refining the particles through mechanochemical treatment.
  • the hydraulic alumina and zircon are ground and mixed almost uniformly through the mechanochemical treatment, and thus boehmite and zircon are firmly fixed in the test sample obtained.
  • the test sample gives good results in both flexural strength and Vickers hardness. From FIGS. 3 and 16 , it becomes possible to further enhance the flexural strength and Vickers hardness of the obtained test sample by including zircon as inorganic oxide particles.
  • hydraulic alumina BK-112 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared.
  • alumina (Al 2 O 3 ) advanced alumina AA-03 manufactured by SUMITOMO CHEMICAL COMPANY, LIMITED was prepared. Note that the advanced alumina AA-03 is a single crystal particle of ⁇ -alumina having a shape close to a polyhedral spherical shape, and the center particle size is 0.40 ⁇ m.
  • the hydraulic alumina and alumina powder were each weighed to be 50% by volume, and the hydraulic alumina and alumina powder were subjected to mechanochemical treatment using a planetary ball mill. Specifically, the hydraulic alumina, alumina powder, ethanol as a solvent, and hard balls (media) were put into a resin pot. Note that the pot used was made from polyamide and had a volume of 250 mL. The media used were made from zirconia and had a diameter of ⁇ 3 mm.
  • the hydraulic alumina was ground by treating, with a planetary ball mill, the pot including the hydraulic alumina, alumina powder, solvent and hard balls.
  • the planetary ball mill was the same as that of example 1-1.
  • the rotation speed of the pot was set to 250 rpm, and the treatment time was set to 3 hours.
  • the mixture of the hydraulic alumina, alumina powder, and solvent was taken out from the pot, and the solvent was removed to obtain a mixture of hydraulic alumina and alumina powder which were mechanochemically treated.
  • the mixture of hydraulic alumina and alumina powder was mixed with the ion-exchanged water using an agate mortar and pestle to obtain a mixture.
  • the mixture obtained was put into a cylindrical mold ( ⁇ 10) having an internal space. The mixture was then heated and pressurized under a condition of 400 MPa, 180° C., and 20 minutes to obtain a test sample of example 6-1.
  • a test sample of example 6 - 2 was obtained in the same manner as in example 6-1 except that cordierite (2MgO ⁇ 2Al 2 O3 ⁇ 5SiO 2 ) was used instead of alumina (advanced alumina AA-03).
  • cordierite 2MgO ⁇ 2Al 2 O3 ⁇ 5SiO 2
  • SS-200 average particle size 7.5 ⁇ m
  • MARUSU GLAZE Co., Ltd. was used as MARUSU GLAZE Co., Ltd.
  • Flexural strength was measured in accordance with JIS T6526 for the test samples of example 6-1 and example 6-2. Consequently, as illustrated in FIG. 17 , the flexural strength of the test sample of example 6-1 was 103.2 MPa. The flexural strength of the test sample of example 6-2 was 90.7 MPa.
  • Vickers hardness was measured in accordance with JIS R1610 for the test samples of example 6-1 and example 6-2. As illustrated in FIG. 17 , the Vickers hardness of the test sample of example 6-1 was 2.6 GPa. The Vickers hardness of the test sample of example 6-2 was 1.9 GPa.
  • FIG. 18 illustrates the result of observing the cross section of the test sample of example 6-1 using a scanning electron microscope. It is evident from FIG. 18 that the boehmite particles 2 made from the boehmite phase are an indefinite shape crystal in the test sample. That is, it is evident that the boehmite particles 2 have many irregularities on the surface and the shape is not constant. In contrast, it is evident that the inorganic oxide particles 4 made from a-alumina have a particle shape and further have faceted surfaces on the particle surface.
  • the present disclosure provides a boehmite structure having high strength and a method for producing the boehmite structure.

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